The burnt gases from internal combustion engines contain numerous pollutants that need to be treated or filtered before these burnt gases are discharged into the atmosphere. Today's challenge in particular is to reduce, not only the emissions of carbon monoxide and of unburnt hydrocarbons, but also the emissions of pollutant soot (pollutant particles) and oxides of nitrogen.
The exhaust line of an engine for this purpose comprises an oxidation catalytic converter followed, in the direction in which the burnt gases flow, by a particulate filter.
The oxidation catalytic converter is coated with a catalytic material intended to store the oxides of nitrogen and, in the presence of oxygen, allow carbon monoxide and the unburnt hydrocarbons to be oxidized. The build-up of oxides of nitrogen on the catalytic material decreases the efficiency of the oxidation catalytic converter.
The particulate filter for its part is designed to filter and store polluting particles. The build-up of particulates in the filter impedes the removal of the burnt gases, causing an increase in the pressure of the burnt gases that is detrimental to the operation of the internal combustion engine.
There are two conventional known techniques for treating the pollutant particles and the oxides of nitrogen that have become trapped in the catalytic converter and in the filter.
A first technique is to inject fuel into the exhaust line during a filter regeneration phase, this leading to a highly exothermic oxidation reaction of the fuel in the oxidation catalytic converter. The burnt gases therefore leave the oxidation catalytic converter at a high temperature (of the order of 650° C.) and enter the particulate filter, burning off the pollutant particles with which the latter is filled.
A second technique is to treat the oxides of nitrogen continuously, during normal engine operation. To do this, an aqueous solution of urea is injected into the exhaust line, upstream of the oxidation catalytic converter, using an injector positioned on this exhaust line. This aqueous solution of urea, upon contact with the burnt gases, is converted into ammonia which reacts in the catalytic converter with the oxides of nitrogen.
Whatever the technique used, it is known practice to position an injector of reducing agent (fuel or urea) at the middle of a duct of the exhaust line in such a way that this injector disperses the reducing agent uniformly into the burnt gases. However, creation of the injector, for reasons associated with the thermal integrity of the injector, entails the use of strong and expensive materials.
Another known solution is to inject the reducing agent at an elbow in the exhaust line. Only the tip of the injector then opens into the pipe, which means that the body of the injector is not subjected to excessively high temperatures.
However, in such devices, because the diameters of the pipes are generally small, some of the reducing agent is sprayed onto the walls of this pipe. This proportion of the reducing agent then flows along the pipe, partially evaporating before entering the oxidation catalytic converter. The mixture of burnt gases and of reducing agent is therefore not uniform; on the contrary, most of the evaporated reducing agent is mixed with the burnt gases flowing along close to the walls of the pipe. As a result, the oxidation reactions occur predominantly near the walls of the oxidation catalytic converter. The temperature of the burnt gases is therefore moderately high at the middle of the catalytic converter and extremely high along its walls, carrying the risk of damaging and of prematurely lessening the performance of this catalytic converter. This problem also occurs identically in the particulate filter which then runs the risk of cracking. Moreover, the oxides of nitrogen and the particulates trapped at the middle of the catalytic converter and of the filter are neither treated nor removed.
There is therefore a difficulty in controlling the homogenization of the reducing agent in the burnt gases in order to optimize the oxidation and combustion reactions that take place in the catalytic converter and in the particulate filter.
Document WO 2006/009056 discloses a device comprising, upstream of the injector, a static helix which causes the burnt gases to swirl in the pipe and become homogenized. However, this intrusive component leads to pressure drops that are detrimental to engine efficiency. The design and production of such a component are also difficult because the helix has to have a very good ability to withstand temperature and vibration. A helix such as this appreciably increases the cost of the exhaust line which has also to comprise flanges intended to hold the helix in place. These flanges not only make the pipe heavier, and therefore more sensitive to vibration, but also introduce problems with heat losses. Finally, a helix such as this has of necessity to be installed in a straight part of the exhaust line, and this considerably increases the amount of space occupied by the exhaust line.